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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_dll.f_a_q_"></a><a class="link" href="f_a_q_.html" title="F.A.Q.">F.A.Q.</a>
</h2></div></div></div>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> Is Boost.DLL thread-safe?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> Some platforms allow concurrent
                calls to <code class="computeroutput"><span class="identifier">dlopen</span></code> like
                functions. For those platforms Boost.DLL is safe in the manner as
                all the C++ Standard Library containers are: it is safe to use different
                instances of shared_library from different threads even if all the
                instances loaded the same library. On other platforms it is not safe
                to concurrently call any of the functions from Boost.DLL (even a
                <code class="computeroutput"><span class="identifier">shared_library</span><span class="special">::</span><span class="identifier">location</span><span class="special">()</span></code>
                call triggers a race condition). See <a class="link" href="limitations.html#boost_dll.limitations.multithread" title="Thread safe library loading (Fails on FreeBSD, MacOS, iOS and some other)">Limitations,
                Thread safe library loading</a>.
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> Why on Linux symbols from one
          plugin are seen in another? Can't get symbol with same name from right
          plugin!
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> You've run into the symbol
                shadowing problem. Compile your plugins with "-fvisibility=hidden"
                flag and take a look to the Tutorial section.
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> How Unicode (Windows) is handled?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> Boost.DLL supports Unicode,
                so that you could provide Unicode paths to it.
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> Can I open an executable file?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> Yes, you can. Symbols need
                be exported using in the executable using <code class="computeroutput"><span class="identifier">BOOST_SYMBOL_EXPORT</span></code>
                or <code class="computeroutput"><span class="identifier">BOOST_DLL_ALIAS</span></code>.
                You can call <code class="computeroutput"><span class="identifier">shared_library</span><span class="special">(</span><span class="identifier">program_location</span><span class="special">())</span></code> to load yourself. Refer to the
                Tutorial section for more info. You can also query executables, just
                provide a path to the executable to <code class="computeroutput"><span class="identifier">library_info</span></code>
                class.
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> What if I specify wrong type
          in <code class="computeroutput"><span class="identifier">shared_library</span><span class="special">::</span><span class="identifier">get</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
          or <code class="computeroutput"><span class="identifier">import</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> Usually you'll end up with
                <code class="computeroutput"><span class="identifier">Segmentation</span> <span class="identifier">Fault</span></code>.
                However it is safe to make types more strict, for example making
                <code class="computeroutput"><span class="keyword">const</span> <span class="keyword">int</span></code>
                from an <code class="computeroutput"><span class="keyword">int</span></code> will not
                harm.
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> Does your library guarantee
          ABI stability of methods?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> Library only guarantees
                that alias names created using the <code class="computeroutput"><span class="identifier">BOOST_DLL_ALIAS</span></code>
                macros will not change with the change of compiler or platform. You
                must take care of functions ABI and API stability by your own.
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> Are there any function signature
          restrictions for the exported/imported functions?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> No. You may import/export
                functions with any signature and any return parameter.
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> I have 2 plugins that use same
          shared library. Would be the shared library loaded twice?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> No. Pugins will share the
                shared library instance.
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> I have 2 plugins each of them
          must work with it's own version of <code class="computeroutput"><span class="identifier">libsome_library</span></code>
          shared library. How to achieve that?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> Avoid such situations by
                statically linking in the libsome_library into each plugin and loading
                plugins with <code class="computeroutput"><span class="identifier">load_mode</span><span class="special">::</span><span class="identifier">rtld_deepbind</span></code>.
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> Is there a <code class="computeroutput"><span class="identifier">RTLD_NODELETE</span></code>
          mode? Is there a built in way to skip unload or "leak" a dynamically
          loaded library?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> No. Boost tries to support
                best practices and leaking resources is not one of them. If you are
                dynamically loading a third party library that fails to unload, try
                the following:
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">shared_library</span><span class="special">&gt;&amp;</span> <span class="identifier">late_unload_registry</span><span class="special">()</span> <span class="special">{</span>
    <span class="keyword">static</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">shared_library</span><span class="special">&gt;</span> <span class="identifier">registry</span><span class="special">;</span>
    <span class="keyword">return</span> <span class="identifier">registry</span><span class="special">;</span>
<span class="special">}</span>
</pre>
                and push the library to that registry <code class="computeroutput"><span class="identifier">late_unload_registry</span><span class="special">().</span><span class="identifier">emplace_back</span><span class="special">(</span><span class="identifier">third_party_shared_library</span><span class="special">)</span></code>. It will force the library to not
                unload till the end of program.
              </li></ul></div>
        </li></ul></div>
<p>
      If the above <code class="computeroutput"><span class="identifier">late_unload_registry</span><span class="special">()</span></code> does not help - ask the vendor of the library
      for support to dynamically load/unload the library.
    </p>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> How to load a shared object
          from memory??
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> All existing OS avoid loading
                shared libraries directly from userspace memory, so you'll find no
                syscall for such case. Currently Boost.DLL provides no means for
                honest loading shared objects from memory. This requires reimplementing
                dynamic linker logic in userspace for all the platforms, which is
                a huge amount of work and very error-prone. However working patches
                are welcomed!
              </li></ul></div>
        </li></ul></div>
<p>
      Workaround would be to write plugin into a temporary file in RAM and load plugin
      from it:
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">filesystem</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">;</span>

<span class="identifier">dll</span><span class="special">:</span><span class="identifier">shared_library</span> <span class="identifier">load_from_memory</span><span class="special">(</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span><span class="special">&amp;</span> <span class="identifier">tmp_plugin_path</span> <span class="special">=</span> <span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">unique_path</span><span class="special">()</span> <span class="special">/</span> <span class="string">"libplugin.so"</span><span class="special">)</span> <span class="special">{</span>
    <span class="keyword">const</span> <span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span> <span class="identifier">plugin_location</span> <span class="special">=</span> <span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">temp_directory_path</span><span class="special">()</span> <span class="special">/</span> <span class="identifier">tmp_plugin_path</span><span class="special">;</span>
    <span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">create_directories</span><span class="special">(</span><span class="identifier">plugin_location</span><span class="special">.</span><span class="identifier">parent_path</span><span class="special">());</span>
    <span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">ofstream</span> <span class="identifier">ofs</span><span class="special">(</span><span class="identifier">plugin_location</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ios</span><span class="special">::</span><span class="identifier">out</span><span class="special">|</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios</span><span class="special">::</span><span class="identifier">bin</span><span class="special">|</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios</span><span class="special">::</span><span class="identifier">trunc</span><span class="special">);</span>
    <span class="identifier">ofs</span><span class="special">.</span><span class="identifier">write</span><span class="special">(</span><span class="identifier">data</span><span class="special">,</span> <span class="identifier">size</span><span class="special">);</span>

    <span class="keyword">return</span> <span class="identifier">dll</span><span class="special">::</span><span class="identifier">shared_library</span><span class="special">(</span><span class="identifier">plugin_location</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
    </p>
<p>
      But there's no guarantee that <code class="computeroutput"><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">temp_directory_path</span><span class="special">()</span></code> will actually write to RAM, that's very platform
      dependent.
    </p>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> Can I use function pointer type
          (like <code class="computeroutput"><span class="keyword">void</span><span class="special">(*)()</span></code>)
          instead of function type (like <code class="computeroutput"><span class="keyword">void</span><span class="special">()</span></code>) to import function?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> No you can't because the
                meaning of importing a function pointer is different:
                <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: square; ">
<li class="listitem">
                      <code class="computeroutput"><span class="identifier">lib</span><span class="special">.</span><span class="identifier">get</span><span class="special">&lt;</span><span class="keyword">void</span><span class="special">()&gt;(</span><span class="string">"func"</span><span class="special">)</span></code>
                      imports a function <code class="computeroutput"><span class="keyword">void</span>
                      <span class="identifier">func</span><span class="special">()</span></code>
                    </li>
<li class="listitem">
                      <code class="computeroutput"><span class="identifier">lib</span><span class="special">.</span><span class="identifier">get</span><span class="special">&lt;</span><span class="keyword">void</span><span class="special">(*)()&gt;(</span><span class="string">"func_var"</span><span class="special">)</span></code>
                      imports a variable that stores a pointer to the function <code class="computeroutput"><span class="keyword">auto</span> <span class="identifier">func_var</span>
                      <span class="special">=</span> <span class="special">&amp;</span><span class="identifier">func</span></code>
                    </li>
</ul></div>
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> I have found a bug, how do I
          notify?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> Create an <a href="https://github.com/apolukhin/Boost.DLL/issues" target="_top">issue
                at GitHub</a> with a detailed description.
              </li></ul></div>
        </li></ul></div>
</div>
<div class="copyright-footer">Copyright © 2014 Renato Tegon Forti, Antony Polukhin<br>Copyright © 2015 Antony Polukhin<br>Copyright © 2016 Antony Polukhin, Klemens Morgenstern<br>Copyright © 2017-2025 Antony
      Polukhin<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
      </p>
</div>
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